I have only used Zecco to buy some limited amounts of stock.
The difference in most of the online brokerages is the customer service, features, or tools available to you. Basically you get what you pay for.
Zecco is/was pretty low frills and has inexpensive trades.
The majority of the sites you see advertised on TV are probably middle of the road – Scottrade, Etrade, etc. Pay a little more but have more tools.
Little higher end would be Charles Schwab, etc… where you can actually go into one of their branches and talk to someone if you have to.
I’ve had no problem with my Zecco account.